From the looks of it, they're leveraging a fork of a library I ported from Compiz a few years ago (called libanimation). I'm quite happy to see this, as I thought it was a dead end and wouldn't get any users, but it looks like its been very useful to this project and I'd be interested to see where the development is continuing so that I can help them out.
Wobbly windows is such a truly excellent feature; I really don’t understand why more platforms (most significantly Windows and macOS) don’t go with it, even if more restrained (Zorin’s jelly looks a bit too loose for my liking). They make the entire experience much more tactile and make windows all round much nicer to work with.
(For my part, I now use tiling window managers, where wobbly windows won’t work well except for floating windows. But on all-floating window managers—)
